Electrophotographic (EP) laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (electrically conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. The image quality can be enhanced by a large number of approaches, including the technique which utilizes small particle developer including dry toner having an average particle size less than 5 .mu.m; see, e.g., U.S. Pat. Nos. 4,927,727; 4,968,578; 5,037,718; and 5,284,731. However, it has also been known that the electrophotographic dry toner having particle size less than 1 .mu.m is very hard to prepare due to increased specific area, and consequently, liquid toner has become one of the solutions for practical preparation of submicrometer xerographic developer.
Liquid toners comprise pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image.
Recently, there has been an increased demand of environmental safety. The industrial response to this requirement has been the investigation of safer solvents for organic coatings. However, in the field of the photoconductor technology, the use of non-chlorinated solvents requires overcoming some challenges in the formulation of the photoconductors, because in the many photoconductor products comprising organic coatings, the best performance is easily achieved with chlorinated solvents, including the stable dispersion of organic pigments and dyes, the uniformity of the coating due to the best compatibility between the photoconductor elements, and the optimum solubility of the binder when the coating solution is made of chlorinated solvents. Thus, there is a need to combine the appropriate photoconductor elements in a non-chlorinated solvent-coating formulation so that the basic performance of the photoconductor can be achieved.
Thus, binders which exhibit satisfactory dispersion performance of the meta-stable pigment crystal forms are not always available when the non-chlorinated solvents are used.
Description of Dual Layer OPC
The organic photoconductor products in the market today, generally speaking, are dual layer OPCs, which comprise a charge generation layer (CGL) and a charge transport layer (CTL) as key components. In addition to these layers, the photoconductor body can be undercoated or overcoated with other materials to improve adhesion to the substrate or to improve surface wear resistance or to reduce the surface adhesion for improved image transfer efficiency. The organic photoconductor (OPC) with an additional undercoating layer or overcoating layer becomes an organic photoreceptor (OPR) and ready for use in various designs of electrophotographic systems.
Most of the multilayer OPRs in the market are negative charging OPCs in which the thick hole transport layer is located on the top of the thin CGL. This is called the standard, or conventional, dual layer OPC. In the conventional case, the CGL usually comprises a photoconductive pigment or dye dispersed in an inert binder, with a pigment/dye content ranging up to about 90 wt%. 100% pigment in the CGL is possible where the pigment CGL is vacuum-evaporated in the format of a thin film; see, e.g., U.S. Pat. No. 4,578,334. Besides dispersion stabilizing functions, the CGL binder also plays an important role of adhesion.
The choice of CGL binder in the conventional dual layer OPC is not very critical, because the CGL is very thin and the binder content is less than 50 wt% in general to ensure a good contact between charge generator (pigment or dye) and charge transport molecule. The good contact between charge generation molecule (CGM) and charge transport molecule (CTM) is the most critical requirement for the high efficiency of charge generation and charge injection of the photoinduced carriers from CGL into CTL if the ionization potential of the charge generation molecule and the charge transport molecule are well-matched and if the electric field crossed over between the two layers is high enough to cause the charge generation, the charge injection, and the charge transport actions.
In reality, the "good contact" between CGM and CTM of a conventional dual layer OPC is formed during the coating of the CTL on the CGL, because the thicker CTL coating needs longer drying time and the coating solvent has an opportunity to create a mixing zone at the CTL/CGL interface due to the slight solubility of the pigment or dye charge generation molecules in the CTL coating solvent. It has been known that the chlorinated solvents, such as dichloromethane (DCM), trichloroethane (TCE), etc., offer the best performance for the formulation of conventional dual layer OPC for two reasons: (1) chlorinated solvents are the best choice for the solubility of most of the binders which can be used for the CTL, such as polycarbonates, and (2) they are also able to create a "slight dissolving" of the pigment or dye CGMs required for forming a mixing zone of CGL/CTL.
Problems of Inverted Dual Layer OPCs
In contrast to the conventional dual layer OPCs for negative charging, an inverted dual layer OPC utilizing the hole transport molecule in the CTL is employed to provide the positive charging OPCs.
In this case, the CGL is deposited on the top of the CTL. Due to the fact that the thinner CGL coating requires much less amount of coating solution and the CGL coating can be dried faster, then the mixing zone of CTM and CGM is harder to form in an inverted dual layer OPC. Thus, the speed of an inverted dual layer OPC becomes poorer than the conventional dual layer OPC, especially when the CGL coating is derived from a non-solvent of the CTL. The situation becomes worse when non-chlorinated solvents are used for forming the coatings on a substrate, because many polymers show poorer solubility in non-chlorinated solvents than in chlorinated solvents. "Better contact" (in the mixing zone) can be achieved by increasing the CGM pigment or dye content in the CGL, for example, above 50 wt%, as disclosed in U.S. Pat. No. 4,948,687. When the solid percentage of pigment or dye CGM in CGL is above 50 wt%, the volume percent can reach the level of 60 to 70 vol %, depending on the density of CGM. Then, there are several issues related to high CGM dispersion coating. First, the poor dispersion stability is caused by the low coverage of dispersion binders on the surface of individual CGM particles. The poor dispersion stability is also caused by the agglomeration or cluster of CGM. Second, the CGM is the most vulnerable component of the photoconductor device, so that the higher the pigment or dye concentration on the surface, the more easily the following disadvantages occur:
(a) surface charge injection, which tends to decrease dark decay with repeat cycle; and PA1 (b) low wear resistance, which reduces the device life and so it is necessary to have a very strong surface protection, which increases the manufacturing cost and reduces productivity; see, e.g., U.S. Pat. Nos. 5,240,802 and 4,409,309. PA1 (a) be soluble in non-chlorinated solvents; PA1 (b) form a stable dispersion with the charge generation molecule (pigment or dye); and PA1 (c) be compatible with the CTM. The poor compatibility between CTM and binder exhibits recrystallization of CTM in a dried film and poorer performance stability. PA1 (a) utilize non-chlorinated solvents for the coating process, including dissolving, milling, mixing, and coating; PA1 (b) achieve excellent dispersion or super dispersion of CGM in CGL and achieve excellent uniformity of the coating; and PA1 (c) achieve comparable speed as the conventional dual layer OPC using the same materials and superior life cycle. PA1 (a) solubility in non-chlorinated solvents; PA1 (b) pigment or dye dispersion stability; and PA1 (c) flexibility of the polymer conformation (as measured by T.sub.g). PA1 (a) solubility in non-chlorinated solvents; PA1 (b) compatibility with transport molecules; and PA1 (c) rigidity of the polymer chain (as measured by T.sub.g).
The addition of CTM into the CGL is one of the solutions to improve the formation of the mixing zone of CGM/CTM in the formulation of the inverted dual layer OPC; see, e.g., U.S. Pat. No. 4,968,579. However, in this case, the selection of CGL binder is more critical because it must simultaneously satisfy three basic requirements:
In order to satisfy the compatibility between CTM and CGL binder, the CGL binder has been chosen to be the same binder as the CTL binder, which is currently and practically a polycarbonate; see, e.g., U.S. Pat. No. 4,968,579. Furthermore, it is observed in many cases, including U.S. Pat. No. 4,968,579, that polymers having a ring in the main chain, such as polycarbonates and polyesters, can provide desirable compatibility with CTM, but they are not able to provide a satisfactory dispersion of pigments or dyes utilized as charge generation molecules. The phenomenon becomes worse when a non-chlorinated solvent is used as a dispersion solvent due to its lower polarity than chlorinated solvents. In this case, a relatively low loading CTM such as 10 wt% or less must be used in order to achieve dispersion and this results in insufficient light absorption efficiency due to the small amount of CTM in CGL. So, in order to achieve enough light absorption efficiency, the device requires relatively thick CGL such as in the range of 10 .mu.m. This kind of thickness easily causes a charge build-up effect due to charge trapping phenomenon in such a heterogeneous phase.
Moreover, the satisfactory dispersion is defined by particle size less than 1 .mu.m in the disperse media after coating finish. The satisfactory dispersion is also determined by the glossiness of the finish coating surface. The agglomeration of dispersed pigment or dye CTM can be observed by evaluation of the glossiness of the coating which has been dried enough, especially when the pigment or dye content in the coating is above 5 wt%: the glossier the coating, the better the dispersion stability. The above-described satisfactory dispersion is called a "super dispersion", which is preferred in order to achieve very low noise and a low graininess image such as the photographic quality achieved by silver halide imaging materials. In this case, the chlorinated coating solvents such as dichloromethane, trichloroethane, and chloroform have been known to facilitate somehow the dispersion quality, even though that dispersion quality is not totally equivalent to a "super dispersion" quality. Of course, these chlorinated solvents are no longer preferred for industrial scale-up due to the environmental concerns mentioned above.
Not only are the super dispersion characteristics required for high image quality, but also the physical arrangement of pigment or dye CGM strongly affects the reliability of the device performance. The agglomeration of the CGM can enhance the positive surface charge injection known as surface charge leak current; see, e.g., U.S. Pat. No. 4,444,862. So, the more uniformly the CGM is dispersed throughout the CGL, the better the performance reliability.
For example, polyvinyl butyral (PVB) is known to exhibit excellent dispersion stability with a number of metastable phthalocyanine pigments, with photoconductive perylene pigments in suitable non-chlorinated solvents such as methyl isobutyl ketone (MIBK), or with tetrahydrofuran (THF), but PVB is not very compatible with most of the well-known hole transport molecules, including hydrazone compounds, triaryl amine compounds, triphenyl methane compounds, and the like. On the other hand, some polycarbonates, such as Makrolon (Mobil Chemical) and polyesters (Vylon Products, Toyobo), exhibit excellent compatibility with the transport molecules, but they do not evidence a good and stable pigment dispersion in non-chlorinated solvents, including THF and toluene. Some non-chlorinated solvents have a tendency to damage the desired crystal structure of some photoconductive pigments and also to reduce the dispersion stability due to the crystal form change during milling processes.
Thus, the main purpose of the present invention is to provide a coating formulation of an inverted dual layer OPC for positive charging with the following benefits: